Methods and systems for detection and identification of concealed materials

ABSTRACT

Methods and systems for efficiently and accurately detecting and identifying concealed materials. The system includes an analysis subsystem configured to process a number of pixelated images, the number of pixelated images obtained by repeatedly illuminating regions with a electromagnetic radiation source from a number of electromagnetic radiation sources, each repetition performed with a different wavelength. The number of pixelated images, after processing, constitute a vector of processed data at each pixel from a number of pixels. At each pixel, the vector of processed data is compared to a predetermined vector corresponding to a predetermined material, presence of the predetermined material being determined by the comparison.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 13/667,706, filed on Nov. 2, 2012, entitled METHODS AND SYSTEMSFOR DETECTION AND IDENTIFICATION OF CONCEALED MATERIALS, which claimspriority to U.S. Provisional Application No. 61/555,804, filed Nov. 4,2011 entitled METHODS AND SYSTEMS FOR DETECTION AND IDENTIFICATION OFCONCEALED MATERIALS, both of which are incorporated by reference hereinin their entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made partially with U.S. Government support from theU.S. Army under contract W31P4Q-09-C-0585. The U.S. Government may havecertain rights in the invention.

BACKGROUND

These teachings relate generally to methods and systems for detectingconcealed materials.

Numerous conventional approaches have been taken in the field ofstandoff detection and identification to attempt to detect and identifymaterials, especially explosives, drugs, etc., concealed under clothing.Such conventional approaches that have been reported in the literaturefor standoff detection and identification of concealed contrabandmaterials include: x-ray backscatter imaging, neutron excited gamma rayemission spectroscopy, terahertz reflection spectroscopy, and laserinduced breakdown spectroscopy.

Problems with the x-ray backscattering imaging approach include: poorchemical selectivity for chemical identification with high potential forfalse positives, large size and weight of instrumentation which preventsthe system from being man-portable, and human health risk from x-rayexposure.

Problems with neutron excited gamma ray spectroscopy include: limitedchemical selectivity resulting from the measurement only producingelemental concentration results, limited sensitivity, and longmeasurement times at significant standoff distances (i.e. 1 ft. orgreater), and substantial human health risks. Measurements providingonly elemental analysis information would not be likely to be able toidentify explosive materials such as triacetaone-triperoxide thatcontain only the elements C, H, and O, and identification of drugs wouldbe very difficult.

Problems with terahertz spectroscopy include: slow measurement time, aswell as substantial problems with interference from absorption ofterahertz radiation by atmospheric water vapor for standoff distancesgreater than 10 ft. In addition, the size and weight of the equipmentare too great for man-portability.

Laser induced breakdown spectroscopy (LIBS) is a trace detection methodthat can detect and identify small particles of explosive or othermaterials on the outside of a surface in a standoff mode. The primaryproblem with LIBS is that it cannot detect or identify materialsconcealed underneath a covering layer such as cloth and can only detectexplosive particles on the outside surface of clothing. Explosives orother contraband materials that are well sealed in a plastic bag andconcealed under clothing, where the outside surface of the clothing wasnot contaminated with the dust of the contraband material, could not bedetected or identified with LIBS.

Further, NIR spectroscopy has been used to identify chemical compounds.In particular, Li, et al. disclose a method of analyzing NIR data, so asto identify various solid forms of chemical compounds and drugcandidates. This method includes the steps of: (1) computing the secondderivative spectra for collected NIR spectra; (2) applying principalcomponent analysis (PCA) of the second derivative spectra atpredetermined wavelengths either the entire wavelength region or aselected wavelength region for segregating the samples; identifying thegroups and group membership from the PCA graph, and further evaluatinggroup members by calculating Mahalanobis distances of a given group toassess qualification of the group members. However, this method ismerely an initial exploratory analysis of near-infrared spectra designedto identify how many different components or materials are present in anunknown sample, and how different their spectra are.

Additional conventional methods include using NIR spectroscopy toattempt to identify components relative to a saved calibration library,via identification of absorption wavelengths, and comparison thereof toknown standards. For example, an explosive device detection method andsystem based on differential emissivity have been disclosed. This methodand system monitors the emissivity levels of target subjects inmonitored zones by repeatedly scanning the pixels of an infraredphotodetector array, and then processing the pixel values to determineif they correspond to at least one calibrated emissivity levelassociated with a concealed explosive device. The calibration techniquesof that method involve attempts to eliminate the effects of clothing andother personal items, as well as environmental factors, but suffer froma concentration mainly on differences in emissivity levels caused bydistance of the target from the source (IR photodetector), rather thanincreasing the contrast/difference in measured emissivity between thecovering materials and the concealed contraband materials.

Further, such conventional methods are inaccurate, when used to attemptto identify materials concealed under clothing, covering materials,etc., due to the difficulties inherent in filtering out the wavelengthsreflected from the clothing, covering materials, containment materials,etc., as well as, importantly, ambient light, sunlight, etc. Thus, toobtain accurate measurements, such conventional NIR methods generallyare confined to laboratory or laboratory-like environments, not publicareas, such as airports.

In view of the above, there is a need for providing a method toefficiently and accurately detect and identify concealed materials, suchas explosives, drugs, or hazardous materials, concealed on a personunder clothing or in a backpack, or concealed in unattended paper,plastic, cloth or leather bags (including backpacks), and a system forcarrying out same.

BRIEF SUMMARY

Methods and systems for efficiently and accurately detecting andidentifying concealed materials are presented below.

In one or more embodiments, the system of these teachings includes anumber of electromagnetic radiation sources, each eletromagneticradiation source having substantially one wavelength from a number ofwavelengths, at least some of the number of wavelengths substantiallycoinciding with wavelengths in an absorption spectrum of predeterminedmaterials, a pixelated image capture device operatively disposed toreceive an image of a region after illumination of the region by oneelectromagnetic radiation source from the number of electromagneticradiation sources, an analysis subsystem configured to process a numberof pixelated images, the number of pixelated images obtained byrepeatedly illuminating regions with a electromagnetic radiation sourcefrom the number of electromagnetic radiation sources, each repetitionperformed with a different wavelength, the number of pixelated images,after processing, constituting a vector of processed data at each pixelfrom a number of pixels, and to compare, at each pixel, the vector ofprocessed data to a predetermined vector corresponding to apredetermined material, presence of the predetermined material beingdetermined by the comparison.

In one or more embodiments, the method of these teachings includesprocessing a number of pixelated images, the number of pixelated imagesobtained by repeatedly illuminating regions with one electromagneticradiation source from a number of electromagnetic radiation sources,each electromagnetic radiation source having substantially onewavelength, each repetition performed with a different wavelength, thenumber of pixelated images, after processing, constituting a vector ofprocessed data at each pixel from a number of pixels, and comparing, ateach pixel, the vector of processed data to a predetermined vectorcorresponding to a predetermined material, presence of the predeterminedmaterial being determined by the comparison.

A number of other embodiments of the system and a method of theseteachings are also disclosed.

For a better understanding of the present teachings, together with otherand further needs thereof, reference is made to the accompanyingdrawings and detailed description and its scope will be pointed out inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1c show a background image (1 a), laser image for L1 (1 b) andL3 (1 c);

FIG. 2 shows a histogram for ratio L1/L3;

FIGS. 3a-3c show a mapped ratio image for L1/L3 (3 a) L2/L3 (3 b), andL4/L3 (3 c);

FIG. 4 shows an example vector in 3D vector space;

FIG. 5 shows an embodiment of the system of these teachings;

FIGS. 6a-6d show block diagram representations of the embodiments of thesystem of these teachings;

FIG. 7 shows a high-level block diagram of the electronics in theexemplary embodiment;

FIG. 8 shows an electrical and software block diagram of the exemplaryembodiment;

FIGS. 9a-9b show a portion of a portable embodiment of the system ofthese teachings;

FIGS. 10a and 10b show embodiments of the method of these teachings;

FIG. 11 shows a block diagram representation of another embodiment ofthe system of these teachings;

FIGS. 12a-12c show a portion of further embodiments of the system ofthese teachings; and

FIG. 13 shows the detecting component in one embodiment of the system ofthese teachings.

DETAILED DESCRIPTION

The following detailed description presents the currently contemplatedmodes of carrying out these teachings. The description is not to betaken in a limiting sense, but is made merely for the purpose ofillustrating the general principles of these teachings.

As used herein, the singular forms “a,” “an,” and “the” include theplural reference unless the context clearly dictates otherwise.

Except where otherwise indicated, all numbers expressing quantities ofingredients, reaction conditions, and so forth used in the specificationand claims are to be understood as being modified in all instances bythe term “about.”

In order to elucidate the present teachings, the following definitionsare provided.

A “projection,” as used herein, is a measure of a portion of a number ofvalues (sometimes referred as a vector) located along another number ofvalues (sometimes referred to as another vector).

An “optical combiner,” as used herein is a passive device in whichemission from several sources (fibers in one embodiment) is distributedto one combination fiber.

In one or more embodiments, the system of these teachings includes anumber of electromagnetic radiation sources, each electromagneticradiation source having substantially one wavelength from a number ofwavelengths, at least some of the number of wavelengths substantiallycoinciding with wavelengths in an absorption spectrum of predeterminedmaterials, a pixelated image capture (also referred to as a detectioncomponent) device operatively disposed to receive an image of a regionafter illumination of the region by one electromagnetic radiation sourcefrom the number of electromagnetic radiation sources, a analysissubsystem configured to process a number of pixelated images, the numberof pixelated images obtained by repeatedly illuminating regions with aelectromagnetic radiation source from the number of electromagneticradiation sources, each repetition performed with a differentwavelength, the number of pixelated images, after processing,constituting a vector of processed data at each pixel from a number ofpixels, and to compare, at each pixel, the vector of processed data to apredetermined vector corresponding to a predetermined material, presenceof the predetermined material being determined by the comparison.

In one instance, each one of the number of the electromagnetic radiationsources sequentially illuminates an area of interest and the number ofelectromagnetic radiation sources emit substantially from one location.The pixelated image capture device (also referred to as a detectingcomponent) receives reflected/scattered electromagnetic radiation fromthe area of interest.

In one or more instances, the analysis subsystem (also referred to as acomponent) includes a background subtraction subcomponent configured forsubtracting, at each pixel from the number of pixels, a background imagepixel value from a pixel value for detected reflected/scatteredelectromagnetic radiation, the background subtraction subcomponentproducing a background subtracted value at said each pixel, a ratiointensity subcomponent configured for obtaining, at each pixel, a numberof ratio values, each ratio value being a ratio of a backgroundsubtracted value at one wavelength from the number of wavelengths to abackground subtracted value at a selected wavelengths from the number ofwavelengths, and a projection subcomponent configured for obtaining, ateach pixel, a measure of a portion of the number of ratio values locatedalong predetermined values at the number of wavelengths for saidpredetermined materials.

In other instances, the analysis subsystem (also referred to as acomponent) also includes a normalizing subcomponent configured tonormalize, for each pixel, the background subtracted value at each pixelrespect to a difference between a value for a measure of emission fromthe electromagnetic radiation source used to generate the image and ameasure of background electromagnetic radiation.

In one embodiment, the system of these teachings also includes a timingcomponent providing a signal for initiation of emission from a selectedone of the electromagnetic radiation sources. The timing component alsoprovides the initiation signal for initiating detection by the pixelatedimage capture device

A block diagram representation of an embodiment of the system of theseteachings is shown in FIG. 6a . Referring to FIG. 6a , in the embodimentshown therein, each one of a number of electromagnetic radiation sources10, each electromagnetic radiation source having substantially onewavelength from a number of wavelengths, at least some of the number ofwavelengths substantially coinciding with wavelengths in an absorptionspectrum of predetermined materials, sequentially illuminates, throughand optical subsystems 40, an area of interest. The number ofelectromagnetic radiation sources emit substantially from one location.The scattered/reflected electromagnetic radiation from the area ofinterest is received by the pixelated detector 50. A timing component 45provides the initiation signal for an electromagnetic radiation source10 and for the pixelated detector 50 and an analysis component 55, sothat the pixelated detector 50 captures the scattered/reflectedelectromagnetic radiation resulting from elimination by theelectromagnetic radiation source 10 at substantially one wavelength andthe data from pixelated detector 50 is captured by the analysissubsystem 55. After the data has been collected for all the wavelengthsfrom the number of wavelengths, the data, at each pixel, can berepresented as a vector, data at each wavelength being data at onecomponent of the vector. At each pixel, the vector of processed data iscompared in the analysis subsystem 55, in one instance, by means of aprojection, to a predetermined vector corresponding to a predeterminedmaterial, presence of the predetermined material being determined by thecomparison.

FIG. 6c shows an embodiment of the analysis subsystem 55. Referring toFIG. 6c , in the embodiment shown therein, the analysis subsystem 55includes a background subtraction subcomponent 60, a normalizingsubcomponent 65, a ratio intensity subcomponent 70 and a projectionsubcomponent 75. The background subtraction subcomponent 60 isconfigured for subtracting, at each pixel from the number of pixels, abackground image pixel value from a pixel value for detectedreflected/scattered electromagnetic radiation, the backgroundsubtraction subcomponent producing a background subtracted value at saideach pixel. The normalizing subcomponent 65 is configured to normalize,for each pixel, the background subtracted value at each pixel withrespect to a difference between a value for a measure of emission fromthe electromagnetic radiation source used to generate the image and ameasure of background electromagnetic radiation. The ratio intensitysubcomponent 70 is configured for obtaining, at each pixel, a number ofratio values, each ratio value being a ratio of a background subtractednormalize value at one wavelength from the number of wavelengths to abackground subtracted normalize value at a selected wavelengths from thenumber of wavelengths. The projection subcomponent 75 is configured forobtaining, at each pixel, a measure of a portion of the number of ratiovalues located along predetermined values at the number of wavelengthsfor said predetermined materials (which is equivalent to the definitionof a projection).

In one instance, the system of these teachings also includes anelectromagnetic emission monitoring component. The timing componentprovides the initiation signal for initiating monitoring, using themonitoring component, of electromagnetic emission from the selected oneof the electromagnetic radiation sources.

In one embodiment, emission substantially from one location for theelectromagnetic radiation sources is enabled by means of an opticalsubsystem. In one instance, the optical subsystem has fiber opticpigtails optically coupled to each electromagnetic radiation source andan optical combiner receiving radiation from the fiber optic pigtails.In another instance, the optical subsystem includes a number of dichroicbeam splinters, each dichroic beam splitter receiving electromagneticradiation from one or more of electromagnetic radiation sources and anoptical fiber receiving electromagnetic radiation from the number ofdichroic beam splitters.

In one embodiment, the analysis component includes one or moreprocessors and one or more computer usable media having computerreadable code embodied therein, the computer readable code causing theone or more processors to subtract, at each pixel, a background imagepixel value from a pixel value for detected reflected/scatteredelectromagnetic radiation, subtraction producing a background subtractedvalue at said each pixel, obtain, at each pixel, a number of ratiovalues, each ratio value being a ratio of a background subtracted valueat one wavelength to a background subtracted value at a selectedwavelength from the number of wavelengths and obtain, at each pixel, ameasure of a portion of the number of ratio values located alongpredetermined values at the number of wavelengths for the predeterminedmaterials.

In one instance, the computer readable code also causes the one or moreprocessors to normalize, for each pixel, the background subtracted valueat each pixel with respect to a difference between a value for a measureof emission from one of the electromagnetic radiation sources and ameasure of background electromagnetic radiation.

FIG. 6d shows an embodiment of the analysis component 55. Referring toFIG. 6d , in the embodiment shown therein, one or more processors 120are operatively connected to a component 110 allowing receiving inputfrom the pixelated detector 50 and to computer usable media 130 havingcomputer readable code embodied therein, where the computer readablecode causes the one or more processors to implement the method of theseteachings for detecting concealed objects. In one instance, the one ormore processors 120 are operatively connected by means of a computerconnection component (such as a computer bus) 135.

In one embodiment, the subcomponents of FIG. 6c are configured forperforming their specific function by the computer readable code,embodied in the computer usable media 130, causing the one or moreprocessors 120 to perform the specific function.

In another embodiment, the system of these teachings includes amodulating component that modulates, with respect to time, the emissionof each electromagnetic radiation source. The modulated emission hastime varying and DC component. FIG. 11 shows a block diagramrepresentation of the embodiments including a modulating component. Asshown in FIG. 11, the modulating component 12 is operatively connectedto the electromagnetic radiation source in order to produce a modulatedelectromagnetic radiation emission. The block diagram representationshown in FIG. 11 is not meant to limit the configuration of themodulating component 12 with respect to the electromagnetic radiationsources 10.

FIG. 12a-12c show different configurations of the electromagneticradiation sources 10 and the modulating component 12. It should be notedthat these teachings are not limited to only the embodiments shown inFIGS. 12a-12c . In the embodiment shown in FIG. 12a , the modulatingcomponent is located after the beam combiner 20. In the embodiment shownin FIG. 12b , the modulating component 12 is located before the beamcombiner 20. Some exemplary embodiments, these teachings not beinglimited only to the exemplary embodiments, of the modulating component12 such as that used in FIGS. 12a and 12b are mechanical devices, suchas a chopper wheel (a chopper wheel, in one instance is similar to thefilter wheel in U.S. Pat. No. 7,328,060, Incorporated by referenceherein is entirety and for all purposes, where some of the filters areclear and other filters are completely opaque), electro-optic modulators(for example, the modulators described in U.S. Pat. Nos. 6,330,097,3,719,414, 3,429,636, in Yariv, Optical Electronics, 3rd edition, pp.274-306, ISBN 0-03-070289-5, 1985 and in Hetch, Optics, pp. 314-321,ISBN 0-201-11609-X, 1974, all of which are Incorporated by referenceherein in their entirety and for all purposes), and acousto-optic amodulators (for example, those described in U.S. Pat. Nos. 4,759,613,7,385,749, and in Yariv, Optical Electronics, 3rd edition, pp. 385-401,ISBN 0-03-070289-5, 1985, all of which are incorporated by referenceherein in their entirety and for all purposes). The choice of modulatordepends on availability, the type of electromagnetic radiation sourceused and the case of providing multiple wavelengths. Embodiments inwhich the modulating component 12 is included in the laser cavity, forexample in a Q switched laser, are also within the scope of theseteachings.

FIG. 12c shows an embodiment in which the modulating component 12 isconnected to or is a part of the drive electronics. Diode lasers can bemodulated by modulating the drive current. (See, for example, theseteachings not being limited only to these examples, U.S. Pat. Nos.7,570,680, 5,651,017, 6,072,816, all of which are Incorporated byreference herein in their entirety and for all purposes).

In some instances, direct or indirect sunlight or incandescent light canintroduce noise indeed detection process by producing signals of largemagnitude. In embodiments in which the detecting component includes aphoto detection subcomponent and an electronic readout subcomponent. Thehigh ambient light contribution from direct or indirect sunlight orincandescent light can be countered by use of short image integrationtimes in order to avoid saturation in the electronic readoutsubcomponent. However, the short integration times can present a limitto the amount of scattered light acquired during image capture. In oneembodiment, the detecting component includes a photo detectionsubcomponent receiving the reflected/scattered electromagnetic radiationfrom the area of interest and providing an electrical signal and anelectronic readout subcomponent receiving the electrical signal. Thephoto detection subcomponent is AC coupled to the electronic readoutsubcomponent. AC coupling can eliminate or greatly reduce the DC ambientlight contribution from direct or indirect sunlight or incandescentlight.

FIG. 13 shows an exemplary embodiment of a detecting component includinga photo detection subcomponent 160 (a photo diode in the embodimentshown) providing an electrical signal after receiving electromagneticradiation and an electronic readout subcomponent 170 receiving theelectrical signal, where the photo detection subcomponent 160 is ACcoupled, by means of a capacitor 180, to the electronic readoutsubcomponent 170.

In yet another embodiment, the system of these teachings includes ahousing. In one instance, the housing has a top portion and a handleportion. The top portion has an opening at one end and a sectionextending away from that end. The pixelated detection component (imageacquisition device) is disposed inside the house and optically disposedto receive reflected/scattered electromagnetic radiation from the areaof interest through the opening. The electromagnetic radiation sourcesare optically disposed such that the electromagnetic radiation sourcesilluminate the area of interest through the opening. Weight anddimensions of the housing and components in the housing are selected toenable the housing to be handheld. The housing is operatively connectedto the analysis component and to timing and power components. In oneinstance, the weight of the housing and components in the housing isless than 10 pounds, preferably less than 4 pounds.

FIG. 9a shows a portion of one embodiment of a portable system of theseteachings including a housing. Referring to FIG. 9a , in the embodimentshown therein, the housing 140 has a top portion 150 and a handleportion 170. The top portion has an opening 160 at one end and a sectionextending away from that end. The pixelated detector 50 is disposed inthat housing and optically disposed to receive, either through opening160 and optic components 175 or through another opening 165, thescattered/reflected electromagnetic radiation from the area of interest.The electromagnetic radiation sources are optically disposed, either bybeing this post in the housing 140, as a component 6, or by beingoptically connected by an optical connection 185 to the housing 140,such that the electromagnetic radiation sources illuminate the area ofinterest through the opening. The data and timing signals can beexchanged through an electrical connector 180. A similar connectorprovides power signals.

FIG. 9b shows another embodiment of the housing 140. In the embodimentshown in FIG. 9b , the handle portion is embodied in the top portion150.

The electromagnetic radiation sources 10 used in the embodiments of thesystem of these teachings can be any of a wide range of electromagneticradiation sources, such as, but not limited to, light emitting diodes,lasers, laser diodes and other electromagnetic radiation sources.

The choice of wavelengths in embodiments of the system of theseteachings is determined by an expeditious and efficient system designbased on considerations such as what, components are best suited for theapplication, availability of components and, in some cases, cost ofcomponents. There is no inherent limitation as to the choice ofwavelengths in the embodiments of the system of these teachings.

In order to better illustrate the present teachings, an exemplaryembodiment is disclosed hereinbelow. It should be noted that theseteachings are not limited to this exemplary embodiment and thatnumerical values presented are presented for illustration purposes andnot in order to limit the present teachings.

It should be noted that these teachings are not limited to the choice ofelectromagnetic radiation sources, wavelengths and detecting componentused in the exemplary embodiment.

Although the exemplary embodiment shown hereinbelow relates to detectingexplosives, it should be noted that other materials are also within thescope of these teachings.

The exemplary embodiment of the system of these teachings includes aninfrared camera (an example of a detecting component or imageacquisition component), a shortwave infrared (SWIR) camera in theexemplary embodiment, a set of laser sources (an example ofelectromagnetic radiation sources), laser diodes in the exemplaryembodiment, that are used to illuminate the area under surveillance, anda reference photodetector that monitors the level of laser lightlaunched by the source. In the exemplary embodiment, each laser diodehas substantially a different emission wavelength within the spectralrange about 0.9 to about 2 micron. The number of laser diodes can varyfrom 2 to 10 depending on the level of spectral identification required.The lasers are fired sequentially so that the illuminated area is bathedin light of only substantially one wavelength at a time. The individuallaser diode signals are made to emit from substantially a commonlocation to control the uniformity of illumination in the area undersurveillance. This can be accomplished, in one instance, these teachingsnot be limited to only that instance, using fiber optic pigtailed laserdiodes and a fiber optic combiner or, in another instance, constructinga laser module in which the laser diode beams all fed into a singlefiber optic using a series of dichroic beamsplitters. One embodiment ofthe components of the system of these teachings that ensure thatindividual laser diodes emit from substantially one location is shown inFIG. 5. Referring to FIG. 5, in the exemplary embodiment shown therein,laser diodes (electromagnetic radiation sources) 10 are opticallyconnected, to optical components 15, fiber-optic pigtails in oneembodiment, that provide the emitted electromagnetic radiation to acombiner component 20. A mode homogenizer 44 and a collimator 47 aresubcomponents in the optical subsystem 40. A feedback photodiode(radiation monitoring component) 30 can detect the electromagneticradiation provided by the collimator 47 or, in another embodiment, candetect the electromagnetic radiation provided to the combiner 20.

A block diagram representation of the exemplary embodiment of the systemof these teachings is shown in FIG. 6b . Referring to FIG. 6b , in theexemplary embodiment shown therein, laser diodes 10 provideelectromagnetic radiation through fiber pigtails 15 to a laser combiner20.

Electromagnetic radiation provided to the laser combiner 20 is monitoredby the photodiode 30. The electromagnetic radiation provided by onelaser diode 10 is delivered through the optical component 40 to area ofinterest. The optical component 40 includes a mode homogenizer 44 and acollimator 47. The electromagnetic radiation scattered/reflected fromthe area of interest is collected by the pixelated detection component50 (a shortwave infrared (SWIR) camera in the exemplary embodiment). Thepixelated data is provided to the analysis component 55.

An electronic trigger signal is used to t the laser diodes. A high-levelblock diagram of the electronics in the exemplary embodiment is shown inFIG. 7. The same trigger signal is also used to trigger the capture ofan image with the SWIR camera and the capture of a referencephotodetector 30 reading of the laser's launched power. The image iscomposed of a digital array of numbers representing the intensity of thelight scatter from objects within the area of surveillance for the laserthat was fired during its collection. Each member of the array is, inone instance, not a limitation of these teachings, a 14 bit readingacquired from a pixel of the camera's detector. A background image mayalso be collected to correct for any ambient light contribution to theacquired image. The reference photodetector signal is also digitized andstored along with the data array for that particular laser's image.

The number of members in the array depends on the type of camera beingused. The camera in the exemplary embodiment has an array of 320 by 256pixels; however cameras with larger or smaller arrays could also beused. The image data collected at each of the different wavelengths istreated as an array of numbers throughout the data processing steps usedto generate the final result. The data processing steps are performed ona pixel-by-pixel basis across the collected images. This means that anoperation like background subtraction is performed by subtracting agiven pixel's value from the background image from the correspondingpixel of an image collected with the laser firing. Any operationgenerates a new array which contains the same number of elements as thearray on which it was performed. The new array can be used to produce anew image by converting the value of each element of the array to a greyscale tint.

The laser wavelengths are selected so that a few of them coincide withregions of the spectrum where the material of interest, in one instance,explosives of interest, absorb electromagnetic radiation and otherswhere the explosives have minimal absorption. Image data collected withthe lasers having wavelengths where little to no absorption is observedare used to correct for the distance dependence of reflected lightintensity (ie, for non collimated light intensity drops off inproportion to 1/r² where r is the distance from the light source).

An embodiment of the method of these teachings is shown in FIG. 10a . Inthe embodiment shown in FIG. 10a , the method of these teachingsincludes sequentially illuminating, for a number of exposures, an areaof interest with electromagnetic radiation, each exposure comprisingelectromagnetic radiation at substantially one wavelengths from a numberof wavelengths (step 210, FIG. 10a ), at least some of the number ofwavelengths substantially coinciding with wavelengths in an absorptionspectrum of predetermined materials, at least some exposures from thenumber of exposures being at different wavelengths. At a number ofpixels, and at each exposure, reflected/scattered electromagneticradiation from the area of interest is detected (step 220, FIG. 10a ). Anumber of pixelated images are processed (step 230, FIG. 10a ), thenumber of pixelated images being obtained by the sequentiallyilluminating. The number of pixelated images, after processing,constitutes a vector of processed data at each pixel from a number ofpixels. At each pixel, the vector of processed data is compared to apredetermined vector corresponding to a predetermined material (step240, FIG. 10a ), presence of the predetermined material being determinedby the comparison.

One embodiment of the processing and comparing steps is shown in FIG.10b . In the embodiment shown in FIG. 10b , the processing and comparingsteps include subtracting, at each pixel from the number of pixels, abackground image pixel value from a pixel value for detectedreflected/scattered electromagnetic radiation (step 250, FIG. 10b ), thesubtraction producing a background subtracted value at said each pixel.For each pixel, the background subtracted value at said each pixel isnormalized with respect to a difference between a value for the measureof emission and a measure of background electromagnetic radiation (step260, FIG. 10b ). At each pixel, a number of ratio values are obtained(step 270, FIG. 10b ), each ratio value being a ratio of a backgroundsubtracted normalized value at one wavelength from the number ofwavelengths to a background subtracted normalized value at a selectedwavelengths from the number of wavelengths. At said each pixel, ameasure of a portion of the number of ratio values located alongpredetermined values at the number of wavelengths for predeterminedmaterials is obtained (the measure is a result of the projection of thevector of processed data onto the predetermined vector corresponding toa predetermined material), a presence of the predetermined materialsbeing ascertainable from that measure.

In one instance, the steps of sequentially illuminating and detectingare performed using a handheld device. In one embodiment, sequentiallyilluminating and detecting are performed while scanning the area ofinterest with the handheld device. In another embodiment, sequentiallyilluminating and detecting are performed in a point-and-shoot manner.

The following describes one exemplary embodiment of the data processingsteps taken to generate differential or ratio images and finally amultidimensional vector that can be used to distinguish the presence ofmaterials, explosives in one embodiment, based on their unique opticalabsorption patterns.

It should be noted that other embodiments are within the scope of theseteachings.

Data Processing

Data processing is used to identify those areas of the images wherewavelength specific attenuation has occurred due to the presence of anexplosive. This processing treats the images as a 2-dimensional dataarray and operates on the individual pixel elements of the arrays thatmake up the images to generate new 2-D arrays. The new 2-D arrays can betransformed back into images by mapping the individual pixel values, inone instance, not a limitation of these teachings, over a 256 step greyscale according to the pixel's value.

Step 1 Background Subtraction

The first step in data processing involves subtraction of backgroundambient light. This step involves subtracting the pixel value in thebackground image from the corresponding pixel value in each of the laserilluminated images. The result is a new image array for each wavelengthwherein the pixel values are proportional to only the laser light beingreflected back to the camera.

Step 2 Normalization for Laser Launch Energy

The output power of the laser diodes is only moderately controlled.Rather than providing a strict control over the actual power launched wesimply measure the launch power at the end of the combiner fiber opticthen normalize each background corrected image for the launch level ofthe laser with which the image was collected. Normalization involvesdividing each pixel of the background corrected array with the signalvalue collected from the system's reference photodetector. The result isa new array with the same number of pixels as the background correctedimage, but with each element of the array normalized to the laser outputpower.

Step 3 Calculating Differential or Ratio Intensity Image Data

The presence of an explosive in the area under surveillance would resultin differences in the image data collected with laser wavelengths thatcoincide with absorption bands versus those that do not. Two simple waysto see these differences is to generate differential or ratio images. Adifferential image can be generated by subtracting the pixel value foreach pixel of one image from the corresponding pixel values of anotherimage collected under illumination at a different wavelength. It isimportant that this operation be done on corresponding pixels in the twoimages as each pixel contains data on the reflected light intensity forone specific region of the image plane. Alternatively, a ratio image canbe generated by calculating the quotient of the pixel values for eachpixel of one image and the corresponding pixel values from a secondimage taken at a different wavelength.

Difference images constructed by subtracting background-subtracted andnormalized image data collected at an absorbing wavelength from datacollected at a non absorbing wavelength will appear whiter in any areawhere an explosive is present. This is due to the lower pixel values inthat area of the image where the optically attenuating explosive exists.

Similarly, ratio images constructed by taking the quotient of backgroundsubtracted and normalized image data at an absorbing wavelength and datacollected at a non absorbing wavelength will appear darker in thoseareas of the image where explosives are present.

Step 4 Vector Treatment and Analysis of Image Data

Differential or ratio images can be generated using any uniquecombination of wavelength images collected by the system. The individualpixel values within the multiple image data sets generated by thesetreatments can be used to produce a single vector representation of thecomplete set of images. The vector is calculated by treating eachdifferential or ratio image as a dimension in an n-dimensional spacewherein “n” is the total number of unique difference or ratio images.The projection of the vector along each dimension is defined by thevalue of a pixel within the differential or ratio image data set. Forexample, assume the system is using three (3) wavelengths so there arethree (3) unique ratio image data sets (1/2, 1/3, and 2/3) containingN×M pixels each. A 3-dimensional vector representation of any pixelwithin the three arrays can be then generated by setting the projectionalong each orthogonal dimension equal to the value of the pixel in therespective array. In other words, if you just look at one pixel withinthe array and treat the ratio 1/2 as the x-axis in a 3-dimensional (XYZ)space the value of X in our 3-dimensional space would be equal to thatpixel's value in the 1/2 image data set. We could similarly set thevalue of the same pixel in the 1/3 image data set as the projectionalong the y-axis and the same pixel's value in the 2/3 image data set asthe projection along the z-axis. The data for that pixel could then bedefined as the vector—(X_(1/2), Y_(1/3), Z_(2/3)) wherein the magnitudeof the vector is with respect to the origin. This same calculation canbe run on every pixel in the image data sets for as many uniquecombinations of wavelengths as the user wishes. In some cases it isbetter to not use all the possible permutations, but only a selectsubset. The selection of an optimal set of combinations requiresexperimentation with the spectral characteristics of the explosives ofinterest and spectrum from different potential interfering agents.

The vector that is formed by the spectral results of differential orratio imaging can then be used to determine if an unknown set of imagescontains any of the explosives of interest or not by comparing the pixelvectors (pixel-by-pixel). This process looks at the projection of theunknown image data vectors onto the known explosives vectors. Thiscomparison can look at the direction and magnitude or just direction.The direction is relative to the known explosives vectors (angle betweenthe two vectors). This is easily calculated using the expression:

${\theta = {\arccos\left( \frac{k \cdot u}{{k}{u}} \right)}},$where k•u is the dot product of the known explosive and unknown vectorsand ∥v∥ denotes the magnitude of the vectors (square root sum of thesquares for all the dimensions). Note: previously we defined the“metric” as simply the value of cos(θ). The result will only be zero (ornearly zero) when the two vectors have the same direction (ie, the twovector are from the same type of material).

An alternative treatment of the image data is to digitize it by settinga threshold value above which the differential or ratio is set equal to1 and below which it is set equal to 0. Differential or ratio image datasets can be analyzed in much the same way as the non digitized datasets.

In one embodiment of the system of these teachings, the system includesone or more processors and one or more computer usable media that hascomputer readable code embodied therein, the computer readable codecausing the one or more processors to execute at least a portion of themethod of these teachings. The one or more processors and the one ormore computer usable media are operatively connected.

An electrical and software block diagram of the exemplary embodiment isshown in FIG. 8. Referring to FIG. 8, in the embodiment shown there in,a timing component 45, PIC triggering system, provides the timinginitiation signal (trigger) to the electromagnetic radiation source 10,a laser system, to the detection system 50, an InGaAs camera, and to theanalysis subsystem 55. The detection system 50 provides the data to theanalysis subsystem 55 by means of an input component 110, a framegrabber card, which communicates with the analysis subsystem by means ofa frame grabber DLL. An image analysis library 115 provides, in oneembodiment, the predetermined vector corresponding to a predeterminedmaterial to the analysis subsystem 55.

Step 5 Presenting Results

The vector comparison results can be presented as a grey scale imagelike the simpler differential or ratio image data or thresh holding canbe applied to highlight those areas of the image where the angle betweenthe vectors would indicate a reason for concern (ie, presence ofexplosives identified). In the grey scale approach the absolute vectorangular differences would be translated into a grey scale value whereinthe grey scale values map the angular range of 0 and 180°. Setting thedark end of the grey scale equal to 0° would yield images with darkerregions in the areas of the image where the vector differences were zeroor nearly zero indicating the potential for the presence of explosives.Alternatively, a threshold comparison can be applied to the vectordifferences and only those pixels whose values are very close to zeroassigned a value of 0 and all other pixels assigned a value of 1 (orvice versa). Images generated following this type of treatment would besharply contrasted. Another, perhaps better way, to present the resultswould be to overlay the thresholded image results with a single imageusing red or a colored scale (blue to red) to highlight the values forthe angular difference. The single image could be any one of theoriginal images collected under a single wavelength illumination. Thecolor scale could highlight in red those areas of greatest concern (verylow or zero angular difference between the known and unknown vectors).One advantage to this approach is that the operator would see a fullgrey scaled image of the area under surveillance making it easier toidentify the potential suspect or object holding the explosives.

In order to further better illustrate the present teachings, anexemplary embodiment of the data processing is disclosed hereinbelow.

The purpose of the process is to remove the ambient light effect andhelp to detect the materials, explosives in the exemplary embodiment.The example includes four (4) lasers from which three (3) ratio imagesare generated. A graphic showing how a vector is constructed inthree-dimensional space from the projection of X, Y, and Z components isalso provided. This graphic shows a simulated vector for an explosive(Ve) and a simulated vector for an unknown compound (Vu) having asizeable angular difference between them.

Data Processing Method

1. Normalize the Laser Image with Photo Diode Reading

For each laser image, normalization is done using Equation 1,

$\begin{matrix}\begin{matrix}{\alpha_{i} = \frac{L_{i} - {B\; K}}{{PIN}_{i} - {PIN}_{B\; K}}} & {{i = 1},\ldots\mspace{14mu},6}\end{matrix} & (1)\end{matrix}$where L_(i) is the laser image, BK is the background image, PIN_(i) andPIN_(BK) are photo diode reading for laser and background, respectively.Here background image is the image acquired when no laser diode isturned on. The purpose of background image is to remove the effect ofambient light.

A background image and the raw laser image for L1 and L3 are shown inFIG. 1.

As we can see from the figure, the brightness of image for L1 and imagefor L3 is different. The PIN normalization is to eliminate suchdifference.

In FIG. 1 two DNT pouches are concealed under the two layers of clothes,however, from each single laser image, we cannot detect the DNT pouches.

2 Get the Ratios

After normalization in 2.1, N normalized frames are averaged to get α_(i). The ratio images, are computed using Equation 2.

$\begin{matrix}\begin{matrix}{r_{j} = \frac{{\overset{\_}{\alpha}}_{i}}{{\overset{\_}{\alpha}}_{k}}} & {{j = 1},\ldots\mspace{14mu},4} & {i,{k \in \left\lbrack {1,\ldots\mspace{14mu},6} \right\rbrack}}\end{matrix} & (2)\end{matrix}$

3 Find the Dynamic Ranges

For the ratios obtained in 2.1, the dynamic range is broad. To controlthe dynamic range to exclude outliers, we assume the ratio values areclose to Gaussian distribution, which can be seen from the ratios'shistogram as shown in FIG. 1. The mean r _(j) and standard deviationσ_(j) of the ratio image are computed. Through our experiments, wechoose [ r _(j)−1.5*σ_(j), r _(j)+1.5*σ_(j)] as the dynamic range. Thebenefit is that this dynamic range is computed automatically from theratio image itself and can adapt to the lighting changes.

4 Map the Ratios to Gray Scale Image

With the dynamic range ready, we can map the ratios to gray scale imageas the final output. The mapping is done using equation (3).

$\begin{matrix}{I = {255*\frac{r_{j} - \left( {{\overset{\_}{r}}_{j} - {1.5*\sigma_{j}}} \right)}{3*\sigma_{j}}}} & (3)\end{matrix}$

After the mapping, the ratio computed by equation (2) is mapped to agray scale image and can be detected by human eyes. FIG. 2 shows someratio images for the case when a human subject conceals the DNT pouchunder two layers of clothes.

5 Vectors from Multiple Laser Ratio

To fuse the information from individual ratio, we propose to form avector feature from multiple ratios, as shown in equation (4).v=(r _(i) ,r _(j) ,r _(k)) i,j,k ε[1, . . . ,4]  (4)

In such a way, the individual ratio becomes the component of the vector.Such a vector combines the information from multiple lasers and willhave stronger detection capability than single ratio. An example vectorgiven in equation (4) is shown in FIG. 4, where V_(e) is the vector fromexplosive region and V_(u) is from other regions. There is an angle θbetween the two vectors.

The principle for the vector based detection is: each individual ratiowill generate values different for regions with and without explosivepouches. Therefore, the vectors composed of these ratios in explosiveregion will point to some specified direction with certain magnitude,while vectors without explosive pouches will point to some uncertaindirections and magnitude. In such way, the region with explosive poucheswill be detected.

While the above exemplary embodiment referred to the detection ofexplosives, these teaching are not limited only to detecting explosivesand the method can be applied to other concealed materials.

For the purposes of describing and defining the present teachings, it isnoted that the term “substantially” is utilized herein to represent theinherent degree of uncertainty that may be attributed to anyquantitative comparison, value, measurement, or other representation.The term “substantially” is also utilized herein to represent the degreeby which a quantitative representation may vary from a stated referencewithout resulting in a change in the basic function of the subjectmatter at issue.

The detection method of the present teachings is preferably performed atsome finite distance from the material being detected, which is referredto as the “standoff distance”. The standoff distance could be in therange of from 1 cm to 100 m. In all cases, the material being detectedmay be concealed under some type of covering materials such as cloth,paper, plastic, or leather that has substantial optical absorptionand/or light scattering properties which obscures viewing the concealedmaterial under the covering material with light in the visiblewavelength range (400-700 nm).

Elements and components described herein may be further divided intoadditional components or joined together to form fewer components forperforming the same functions.

Each computer program may be implemented in any programming language,such as assembly language, machine language, a high-level proceduralprogramming language, or an object-oriented programming language. Theprogramming language may be a compiled or interpreted programminglanguage. Each computer program may be implemented in a computer programproduct tangibly embodied in a computer-readable storage device forexecution by a computer processor. Method steps of the invention may beperformed by a computer processor executing a program tangibly embodiedon a computer-readable medium to perform functions of the invention byoperating on input and generating output.

Common forms of computer-readable media include, for example, a floppydisk, a flexible disk, hard disk, magnetic tape, or any other magneticmedium, a CDROM, any other optical medium, any physical medium withpatterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any othermemory chip or cartridge, all of which are non-transitory. As stated inthe USPTO 2005 Interim Guidelines for Examination of Patent Applicationsfor Patent Subject Matter Eligibility, 1300 Off. Gaz. Pat. Office 142(Nov. 22, 2005), “On the other hand, from a technological standpoint, asignal encoded with functional descriptive material is similar to acomputer-readable memory encoded with functional descriptive material,in that they both create a functional interrelationship with a computer.In other words, a computer is able to execute the encoded functions,regardless of whether the format is a disk or a signal.”

Although these teachings has been described with respect to variousembodiments, it should be realized these teachings is also capable of awide variety of further and other embodiments within the spirit andscope of the claims.

What is claimed is:
 1. A system comprising: a detecting componentconfigured to detect incident electromagnetic radiation at a number ofpixels; a number of electromagnetic radiation sources; eachelectromagnetic radiation source emitting at substantially onewavelength from a number of wavelengths; at least some of the number ofwavelengths substantially coinciding with wavelengths in an absorptionspectrum of predetermined materials; each one of the number of theelectromagnetic radiation sources sequentially illuminating an area ofinterest; the number of electromagnetic radiation sources emittingsubstantially from one location; the detecting component receivingreflected/scattered electromagnetic radiation from the area of interest;a modulating component configured to modulate, with respect to time,emission from said each one of the number of electromagnetic radiationsources; a timing component configured to provide a signal forinitiation of emission from a selected one of the number ofelectromagnetic radiation sources; the timing component also configuredto provide said initiation signal for initiating detection by thedetecting component; and an analysis component configured to: process anumber of pixelated images, the number of pixelated images obtained bysequentially illuminating an area of interest, each sequentialillumination being from one of the number of electromagnetic radiationsources, said one emitting at substantially one wavelength from thenumber of wavelengths; processing comprising subtracting, at each pixelfrom the number of pixels, a background image pixel value from a pixelvalue for detected reflected/scattered electromagnetic radiation; thesubtraction producing a background subtracted value at said each pixel;the number of pixelated images, after processing, constituting, at saideach pixel from a number of pixels, a vector of processed data; eachelement of said vector at one pixel being one of a ratio or differentialbetween a value at one wavelength from the number of wavelengths and avalue at another wavelength from the number of wavelengths; at said eachpixel, thereby forming a vector of ratio or differential values at saideach pixel; and compare, at each pixel, the vector of processed data toa predetermined vector corresponding to a predetermined material;presence of the predetermined material being determined by comparing. 2.The system of claim 1 wherein the detecting component comprises aphoto-detection subcomponent receiving the reflected/scatteredelectromagnetic radiation from the area of interest and providing anelectrical signal; and an electronic readout subcomponent receiving theelectrical signal; the photo-detection subcomponent being AC coupled tothe electronic readout subcomponent.
 3. The system of claim 2 whereinthe analysis component further comprises: a ratio intensity subcomponentconfigured for obtaining, at said each pixel, a number of ratio values,each ratio value being a ratio of a background subtracted value at onewavelength from the number of wavelengths to a background subtractedvalue at a selected wavelengths from the number of wavelengths; and aprojection subcomponent configured for obtaining, at said each pixel, ameasure of a portion of the number of ratio values located alongpredetermined values at the number of wavelengths for said predeterminedmaterials; a presence of said predetermined materials beingascertainable from said measure.
 4. The system of claim 3 wherein theanalysis component further comprises a normalizing component configuredto normalize, for said each pixel, the background subtracted value atsaid each pixel with respect to a difference between a value for ameasure of emission from one of the number of electromagnetic radiationsources and a measure of background electromagnetic radiation.
 5. Thesystem of claim 1 further comprising an electromagnetic emissionmonitoring component; wherein the timing component provides saidinitiation signal for initiating monitoring of electromagnetic emissionfrom the selected one of the number of electromagnetic radiationsources.
 6. The system of claim 2 wherein emission substantially fromone location for the number of electromagnetic radiation sources isprovided by use of an optical subsystem.
 7. The system of claim 6wherein the optical subsystem comprises fiber optic pigtails opticallycoupled to each electromagnetic radiation source from the number ofelectromagnetic radiation sources; and an optical combiner receivingradiation from the fiber optic pigtails.
 8. The system of claim 6wherein the optical subsystem comprises a number of dichroic beamsplitters, each dichroic beam splitter receiving electromagneticradiation from one or more of the number of electromagnetic radiationsources; and an optical fiber receiving electromagnetic radiation fromthe number of dichroic beam splitters.
 9. The system of claim 2 furthercomprising: a housing comprising: a top portion; a handle portion joinedto said top portion; said top portion having an opening at an upper end;wherein the detecting component is disposed inside said housing andreceives reflected/scattered electromagnetic radiation from the area ofinterest; and wherein said number of electromagnetic radiation sourcesare optically disposed such that said number of electromagneticradiation sources illuminate the area of interest through said opening;weights and dimensions of the housing and components in the housingbeing selected to enable the housing and components in the housing to behandheld.
 10. The system of claim 9 wherein the detecting componentreceives electromagnetic radiation through said opening.
 11. The systemof claim 9 wherein the detecting component receives electromagneticradiation through another opening.
 12. The system of claim 9 whereinsaid weight is at most 10 pounds.
 13. The system of claim 9 wherein saidelectromagnetic radiation sources are disposed inside said housing. 14.The system of claim 9 wherein said electromagnetic radiation sources areoptically coupled to said housing.
 15. The system of claim 9 whereinsaid handle portion is embodied in said top portion.
 16. The system ofclaim 3 wherein said analysis component comprises: at least oneprocessor; and at least one computer usable medium having computerreadable code embodied therein, the computer readable code causing saidat least one processor to: subtract, at each pixel from the number ofpixels, a background image pixel value from a pixel value for detectedreflected/scattered electromagnetic radiation; subtraction producing abackground subtracted value at said each pixel; obtain, at said eachpixel, a number of ratio values, each ratio value being a ratio of abackground subtracted value at one wavelength from the number ofwavelengths to a background subtracted value at a selected wavelengthfrom the number of wavelengths; and obtain, at said each pixel, ameasure of a portion of the number of ratio values located alongpredetermined values at the number of wavelengths for said predeterminedmaterials; said at least one processor and said at least one computerusable medium constituting the background subtraction subcomponent, theratio intensity subcomponent and the projection subcomponent.
 17. Thesystem of claim 16 wherein the computer readable code further causessaid at least one processor to: normalize, for said each pixel, thebackground subtracted value at said each pixel with respect to adifference between a value for a measure of emission from one of thenumber of electromagnetic radiation sources and a measure of backgroundelectromagnetic radiation.
 18. A method for detecting concealed objects,the method comprising: sequentially illuminating, for a number ofexposures, an area of interest with electromagnetic radiation; eachexposure comprising electromagnetic radiation at substantially onewavelengths from a number of wavelengths; the electromagnetic radiationbeing modulated with respect to time; at least some of the number ofwavelengths substantially coinciding with wavelengths in an absorptionspectrum of predetermined materials; at least some exposures from thenumber of exposures being at different wavelengths; detecting, at anumber of pixels, and at each exposure, reflected/scatteredelectromagnetic radiation from the area of interest; thereflected/scattered electromagnetic radiation from the area of interestbeing detected by a photo-detecting component and an electronic readoutsubcomponent; an output of the photo detecting component being ACcoupled to the electronic readout subcomponent; processing a number ofpixelated images, the number of pixelated images obtained by thesequentially illuminating; each sequential illumination being from oneof the number of exposures; each exposure comprising electromagneticradiation at said substantially one wavelength from the number ofwavelengths; said processing comprising subtracting, at each pixel fromthe number of pixels, a background image pixel value from a pixel valuefor detected reflected/scattered electromagnetic radiation; thesubtraction producing a background subtracted value at said each pixel;the number of pixelated images, after processing, constituting, at saideach pixel from a number of pixels, a vector of processed data; eachelement of said vector at one pixel being one of a ratio or differentialbetween a value at one wavelength from the number of wavelengths and avalue at another wavelength from the number of wavelengths at said eachpixel, thereby forming a vector of ratio or differential values at saideach pixel; and comparing, at each pixel, the vector of processed datato a predetermined vector corresponding to a predetermined material;presence of the predetermined material being determined by saidcomparing.
 19. The method of claim 18 wherein processing and comparingfurther comprises: obtaining, at said each pixel, a number of ratiovalues, each ratio value being a ratio of a background subtracted valueat one wavelength from the number of wavelengths to a backgroundsubtracted value at a selected wavelength from the number ofwavelengths; and obtaining, at said each pixel, a measure of a portionof the number of ratio values located along predetermined values at thenumber of wavelengths for predetermined materials; a presence of saidpredetermined materials being ascertainable from said measure.
 20. Themethod of claim 19 further comprising: monitoring emission for eachexposure; the monitoring providing a measure of emission for said eachexposure; normalizing, for said each pixel, the background subtractedvalue at said each pixel with respect to a difference between a valuefor the measure of emission and a measure of background electromagneticradiation.